Control for bias of magnetic brush and method

ABSTRACT

IMPROVED DEVELOPEMENT OF ELECTROSTATIC CHARGE PATTERNS IS ACHIEVED BY THE USE OF THE DEVELOPMENT CONTROL DEVICE DISCLOSED WHICH ELECTRICALLY BIASES A MAGNETIC BRUSH DURING DEVELOPMENT TO A VOLTAGE INITIALLY SENSED BY THE BRUSH.

July 4, 1972 MORSE 3,674,532

CONTROL FOR BIAS OF MAGNETIC BRUSH AND METHOD Filed July 25; 1970THEODORE H. MORSE INVENTOR.

A TTOR/VEY United States Patent ()fice 3,674,532 Patented July 4, 1972U.S. Cl. 117-17.5 6 Claims ABSTRACT OF THE DISCLOSURE Improveddevelopment of electrostatic charge patterns is achieved by the use ofthe development control device disclosed which electrically biases amagnetic brush during development to a voltage initially sensed by thebrush.

This invention relates to the development of electrostatic chargepatterns and to a novel method and apparatus for development ofelectrostatic charge patterns.

Electrophotographic imaging processes and techniques have beenextensively described in both the patent and other literature, forexample, U.S. Pat. Nos. 2,221,776; 2,277,013; 2,297,691; 2,357,809;2,551,582; 2,825,814; 2,833,648; 3,220,324; 3,220,831; 3,220,833 andmany others. Generally, such processes employ an electrostaticallycharged photoconductive insulating element which responds to imagewiseexposure to electromagnetic radiation by selectively dissipating chargesto form an electrostatic charge pattern. The electrostatic chargepattern is then rendered visible by contacting the charged surface ofthe photoconductive element with suitable developer marking particles.

One development method involves cascading xerographic developer acrossthe image-bearing surface as described in U.S. Pat. No. 2,618,551. Thiscascade development system is adequate for ordinary line copies;however, it has limited application where solid area development isrequired. For solid area development, a magnetic brush developmentsystem has greater utility.

In a magnetic brush development system, a developer mix typicallycomprising ferromagnetic iron carrier granules together with coloredresin toner particles is applied to an electrostatic charge pattern bymeans of an apparatus of the type described in U.S. Pat. No. 3,003,462.In such an apparatus, the iron particles are held by a magnet in abristle-like formation resembling a brush with the toner particlesadhering to the iron by electrostatic attraction. The bristles of ironparticles are electrically conductive and contribute to the transfer oftoner to the charge image-bearing surface.

A magnetic brush of this type is typically electrically grounded, thatis, electrically connected to the machine to which it is mounted. Thisgenerally gives very satisfactory development of charge images in whichthe potential of the background or exposed areas is reduced to zero. Inthe development of charge images in which the background potential hasnot been reduced to zero, considerable density appears in backgroundareas upon development.

This developed density is undesirable and has been overcome in the pastby electrically biasing the magnetic brush at some fixed potential aboveground. Such a system is generally satisfactory where the backgrounddensity of the document to be reproduced is more or less constant. Inmany documents, however, the background may be of variable density, oreven of several colors. Documents of this type do not reproducecorrectly with uniform freedom of background density using a system offixed bias. Consequently, there is a need in the art for a magneticbrush development system which overcomes these disadvantages, which isadapted to give uniformly good reproduction of documents of many types,and which gives freedom from background toning under a wide variety ofconditions.

Copending patent application of A. P. Turner and H. A. Miller, Ser. No.57,654, filed on July 23, 1970, entitled Autobiasing DevelopmentElectrode for Xerography discloses a method and apparatus whichovercomes the aforementioned disadvantages. In that application, uniformdevelopment was accomplished by connecting a series resistance betweenthe magnetic brush and a reference voltage, such as ground. In practice,this system is satisfactory except where the contact area between thebrush and the surface of the print being developed is unavoidably re-'duced, such as at the trailing edge of the print. Since the averagesensed potential is lower in these areas, the brush is not sufiicientlybiased and a band of high density is developed in the background. Thissituation also arises when a dual magnetic brush is used fordevelopment, such as, for example, a brush of the type disclosed incopending patent application of I. B. Ville, Ser. No. 653,'934,-filedJuly 17, 1967, entitled Electrophotographic Developing Method andApparatus. When both brushes are in contact with the surface of theprint being developed, a given potential is induced. After the trailingedge of the print leaves the first brush, the contact area is cut inhalf, and the induced potential is reduced correspondingly.

It is an object of this invention to provide a development method andapparatus which will successfully reproduce documents having a widerange of image and background densities.

It is another object of this invention to provide a method and apparatuswhich will produce copies having little or no background density fromdocuments having a wide variety of background colors.

It is still another object of this invention to provide a magnetic brushdevelopment method and apparatus which controls the background densityof a xerographic reproduction in accordance with the surface potentialof the electrostatic charge pattern sensed during development.

It is a further object of this invention to provide a magnetic brushdevelopment method and apparatus which substantially reducesover-response to variations in sensed surface potential, therebypermitting more nearly uniform development bias to be applied inaccordance with the overall image potential.

The present invention provides, in a xerographic development processwhich utilizes a conductive development electrode and facing and closelyspaced apart therefrom the charge image-bearing surface of anelectrographic element, between which particles of a marking materialare caused to move, which electrode derives its bias potential from apath having a predetermined resistance between itself and a voltagereference source, which may be ground, a method of preventing downwardfluctuations away from an arbitrary voltage of the bias potential soderived from exceeding a predtermined magnitude. The arbitrary voltagemost often used is derived from the surface potential of the elementbeing developed. An apparatus is also provided which senses changes inthe surface potential of the element, compares the potential so sensedwith a stored potential, and supplies an additive corrective potentialwhen needed.

In a preferred embodiment of this invention, the carrier for the markingparticles and the development electrode are the same member, as in thewell-known magnetic brush development system. Other types of electrodedevelopment systems are equally suited to the method of this invention,as will become apparent.

Reference is now made to the drawings, in which:

FIG. 1 is a partially schematic cross-sectional view of a developmentstation suitable for use with the method and apparatus of thisinvention.

FIG. 2 schematically illustrates circuitry for preventing the magnitudeof bias potential from dropping below a predetermined level during thedevelopment of an electrostatic image at the station shown in FIG. 1.

FIG. 3 illustrates schematically further switching whereby the circuitryin FIG. 2 is actuated.

Referring initially to FIG. 1, there is shown in cross section amagnetic brush assembly, generally designated 2, comprising a reservoir4 closed at its ends with covers (not shown) and containing developer 6.Shaft 3 is rotatably secured in the end covers of reservoir 4 and acyllindrical magnet 8, magnetized across a diameter thereof isconcentrically mounted on shaft 3. The rotatable mounting of shaft 3 inthe end covers permits orientation of the magnet for optimum brushformation during the development of electrostatic charge patterns. Alsojournalled on the shaft for rotation thereon are two circular end caps(not shown), between which is concentrically secured a rotatablecylindrical non-ferromagnetic shell 10. The surface of shell ispreferably grooved parallel with its axis of rotation to facilitatecarrying developer along on its surface as it rotates. The end caps andshell 10 completely enclose magnet 8 to prevent contact between themagnet and developer 8. The reservoir 4 is substantially filled with drymagnetic developer 6 suitable for use in magnetic brush development.Shaft 3 is so mounted in the end covers that approximately of the shell10 extends above the average level of developer 6 in reservoir 4.Electrically connected to shaft 3 and reservoir 4 are wires 12 and 14,

respectively, which are brought to a common tie point at the point ofelectrical connection A to the circuit FIG. 2.

In operation, a member to be developed, generally designated 16, ismoved in the direction of arrow 18. Member 16 may comprise a conductivesupport 24 bearing an insulating layer 20 on one surface. The surface oflayer 20 carries the electrostatic charge pattern 22 which is to bedeveloped. Electrically insulated layer 20 comprises a material notsensitive to activating radiation, such as an insulating polymericmaterial, which may additionally contain a substance which is normallyinsulating but which becomes conductive in the presence of activatingradiation, such as a photoconductive material. Shell 10 is customarilycaused to rotate in the direction of arrow 26, counter to the directionof translation of member 16. As it rotates, it carries along on itssurface bristles of the developer 6 which are formed along the lines ofmagnetic flux connecting the N and S poles of magnet 8. As the bristleswipe across the charge pattern 22 on the surface of layer 20, markingparticles (toner) are caused to be removed from the carrier particles ofthe developer in typical fashion and are deposited in accordance withthe charge pattern. During development, an additional potential appearsbetween the surface of shell 10 and the surface of layer 20. It isbelieved that one component of this potential is an induced voltage. Onepossible explanation of the cause of this voltage is now given. When theconductive magnetic brush is brought in close proximity to the chargeimage on the surface layer 20, an induced charge appears on the surfaceof the magnetic brush shell 10. Since there is a resistive element (40,42) between the brush shell 10 and the reference voltage or ground,charges flow into the brush from the source of reference voltage,producing a voltage difference between the brush shell 10 and thereference voltage. The greater the charge density of the image on layer20, the greater the induced charge, thus producing a greater currentflow through resistive elements 40, 42. correspondingly, a greater biasvoltage between the magnetic brush and the reference source is alsoproduced. If the magnitude of the resistive element is increased, thebias voltage due to the current flow through this resistance will alsoincrease. Conversely, if the resistance is decreased, a lower biasvoltage will result. Other contributions to the bias potential mayresult from triboelectric effects and from the removal of toner from thebrush to the image surface. Thus, when the conductive magnetic brush isbrought into close proximity with the charge image on the surface oflayer 20, toner is transferred from the brush to the surface. There thenremains on the brush an excess of charge having a polarity opposite tothat of the lost toner particles. Since there is a resistive element 40,42 between the magnetic brush and the reference voltage or ground, acurrent flows between the brush and the reference source, and a voltageappears across the resistive element as a bias voltage. The greater therate at which toner is removed from the brush, the greater is thevoltage of the brush, and therefore the greater is the bias voltage. Asthe value of the resistance is increased, the charge produced on thebrush is prevented from being neutralized as rapidly, and therefore thebrush assumes a higher voltage. Conversely, as the value of theresistance is decreased, the charge produced on the brush is able to beneutralized more rapidly, and the brush therefore assumes a lowervoltage.

Unlike bias from an external D.C. source, the potential spontaneouslygenerated on the developer brush varies directly with the potential ofthe image area in contact. Thus, in areas representing maximum,intermediate, and minimum densities in the original, electrostatic-imagepotentials of 600, 300 and 75 volts, respectievly, might result inpotentials of the order of 300,130 and 30* volts, respectively, in thecorresponding local surface areas of the developer. Since a considerablearea of the electrostatic image is in contact with the developer at anyone time, the instantaneous average potential, determined, for example,by reading the potential drop across the resistors will represent aweighted average of all the separate potentials on the developer surfacecontacted at that moment. For instance, in a typical xerographic processin which an electrostatic image is developed by traversing a magneticbrush as hereinbefore described, the above range to 300 to 30 voltsmight results in measurements of potential differences across theexternal resistor in the range of from, 200 to volts during thedeveloping period.

An advantage gained from the use of the method of the present inventionfor obtaining a bias potential is that the exposure latitude isincreased and image-free areas remain free of unwanted density whilereproduction of fine-lined etail is maintained.

Since the effective bias is not constant over the entire image area, butdepends on the local instantaneous value of surface potential, theforegoing advantages can be realized.

Effective bias, as used herein, refers to the voltage difference ordifference in potential between the brush and the surface of the elementbeing developed. This is to be distinguished from the customary methodof specifying bias as the voltage between the developing electrode orbrush, and ground. Clearly, it is the effective bias which determineswhether background density will be developed in a given area, or whetherthere will be produced a high enough voltage to cause sparking betweenthe brush and the surface of the element.

A useable portion of the induced voltage appears at point A and is thusapplied across resistors 40 and 42 in series. Proper choice of the valueof resistors 40 and 32 enables preselection of a range of developmentconditions. When the combined resistance is relatively low, e.g., belowabout 10 ohms, the self-bias potential produced is relatively low andvaries only slightly with variations in the surface potential of thelocal area being developed. Relatively narrow exposure latitude resultstogether with good solid-area fill in of those areas havingsubstantially uniform potential. When the resistance is relatively high,e.g., about 10 ohms, the self-bias potential produced is relatively highand highly dependent on the surface potentials of the local areas beingdeveloped. This reduces the spread of effective bias potential values,which causes a lowering of overall contrast and increase in exposurelatitude. Fillin of extended solid areas is then reduced, anddevelopment is largely of the fringing type. Values of resistanceintermediate these extreme conditions produce a continuous gradation andresulting compromise between solid-area fill-in with accompanyingtendency toward producing background density should the exposure not beexactly correct and extreme fringing development with wide exposurelatitude, freedom from background, and poor solid-area fill-in.

' Reference is now made to FIG. 2, which schematically illustrates acircuit to be used in conjunction with a magnetic brush for preventingthe value of bias potential from becoming less in magnitude than apredetermined value. The circuit is connected to the magneticbrush-apparatus at point A.

The circuit of FIG. 2 functions in the hereinafter described manner. Apredetermined fraction of an initial potential produced on the brush bycontact with the charge-bearing surface is impressed on a voltagestorage means. At a predetermined time, the circuit containing thevoltage storage means is disconnected from the brush and the storedvoltage is amplified through a high input impedance amplifier chain bythe amount necessary to make the amplified voltage equal to the initialvoltage. The amplified voltage is compared with the instantaneousvoltage.

It should be noted that the device is not a closed loop, that is, itdoes not use feedback to achieve stabilization, since the input isdisconnected from the brush when the circuit is in operation. This isnecessary because the feedback necessary if a closed loop were usedwould be positive, as can be readily seen. If it is assumed that apositive transient with respect to the reference voltage, or ground,appears on the brush, a positive transient would be fed to theamplifier, amplified, and applied to the brush. If the gain of theamplifier chain is strictly maintained at unity or less, no problemshould occur. However, it is very difficult to keep an amplifieremploying positive feedback stable for more than a few seconds orminutes at the most if the gain of the feedback loop is unity or greaterthan unity. Consequently, an open loop amplifier is preferred.

The operation of the circuit will now be discussed in detail. The valuesof resistors 40 and 32 are selected from values which will provide asample voltage when arranged to form a voltage divider between themagnetic brush and the voltage reference point 44, to sample a smallfixed percentage of the voltage generated by the magnetic brush as itcontacts charge pattern 22. When the polarity of the charge pattern isnegative, the reference is ground, as will be seen hereinafter. As anexample, if the percentage selected were one percent, the value ofresistor 42 would be 1/100 of the value of resistor 40. This voltage isimpressed on capacitor 48 through the normally closed contacts ofsection B of relay 46 which is shown in the nonactuated position. Relay46 permits capacitor 48 to be electrically connectable to the voltagesampling junction of resistors 40 and 42. As can be seen by looking atthe contacts of section C of relay 46, there is no input to amplifier 72under these conditions, so that amplifier gives no effective response tothe input signal.

Reference is now made to FIG. 3 in order to illustrate the operation ofrelay 46. A source of voltage 32 is connected to actuate relay 46 whenswitch 30 is closed. Switch 30 (FIG. 1) is conveniently a spring-returnS.P.S.T. switch, normally open.

Reference is again made to FIG. 2, taken together with FIG. 1. Imagesupport member 16 bearing a pattern of electrostatic charge 22 oninsulating surface is caused to move in the direction of arrow 18 overthe surface of rotating shell 10 of the magnetic brush. In so doing,member 16 contacts the thin layer of developer 6 borne on the magneticbrush surface, causing deposition of marking particles in accordancewith the charge pattern. During development, a potential is producedbetween point A and reference level 44. At a predetermined time, such aswhen leading edge 28 of member 16 actuates switch 30, relay 46 isenergized. The moveable relay blades are then caused to take thepositions shown in dashed lines. Section B of relay 46 disconnects theinput of amplifier 50 from the voltage divider formed by resistors 40and 42, leaving it connected only to capacitor 48. Section A of relay 46connects resistor 41 between the magnetic brush and reference level 44,to prevent unwanted build-up of voltage on the brush which wouldotherwise result from contact with an area of unusually high potentialon surface 20. Section C of relay 46 closes the amplifier chain andpermits the constant po tential stored in capacitor 48 to be impressedon the brush after being suitably amplified.

Amplifiers 50, 64 and 72 are what are known in the analog computer artas operational amplifiers. Such amplifiers are characterised in thatthey can perform addition and subtraction of continuous functions.Typically, they have two inputs and may have one or two outputsdepending on the choice of phase of the output. The output obtained isthe sum of the inputs, or the difference, depending on how the amplifieris connected into its associated circuit. Operational amplifiers arenormally prepackaged, requiring only a power source to supply operatingpotentials and external resistors to control voltage gain and determinethe operation to be performed. Gain is typically controlled by reducingthe effective input voltage by feeding back a certain fraction of theoutput voltage out of phase with the input. Customarily, the gain is theratio of the feedback resistance, such as 68 for amplifier 64, to theinput resistance, or 66 for the same amplifier. If the resistances arethe same, the gain is unity.

Initially, it is assumed that a negative polarity charge 22 is to besensed on the surface of insulating layer 20. The four sections ofswitch 52, a polarity selecting switch, are then in the positions shownin solid lines, the positive output of amplifier 72 is thereby grounded,and amplifier 64 is switched into the circuit. When relay 46 is actuatedas just described, capacitor 48 is disconnected from its input source bythe opening of relay contacts 46B. The input of amplifier 50 is thenheld at a constant potential equal to 1% of the potential sensed by thebrush. Amplifier 50 must have a high input impedance to prevent it fromdischarging capacitor 48 excessively during the interval of operation.In general, its input impedance must be in excess of about 10 ohms inmost situations. The output potential is adjusted by proper selection ofthe values of resistances 58 and 62 and by proper adjustment of variableresistor 60, as well as by appropriate choice of values of +LV and LV.The supply voltages for these latter two may conveniently be madebetween and --10 volts to and -50 volts, with and 15 to 25 volts beingpreferred in the specific circuit described. The voltage gain ofamplifier 50 is controlled by adjustment of feedback resistance 54.Amplifier 50 is a noninverting amplifier and thus does not change thephase of the signal.

The output level of amplifier 50 is then fed to the input ofphase-inverting amplifier 64, which is adjusted to have a gain of unity.Amplifier 64 need not have a high input impedance, as it is not locatedin the circuit in such a place as to discharge any voltage storagemeans. The output of amplifier 64 is then fed through switch section 52Band variable resistor 70 to the input of amplifier 72. Amplifier 72 ischosen to have a voltage gain of up to approximately 200, in order thatan output of 200 volts may be obtained with an input of a volt or two.The particular amplifier shown in the examples has an output capabilityof up to 2000 volts, so resistor 70 is included to attenuate the inputsignal sufiiciently to limit the output to about 500 volts. Amplifier 72has two outputs, one of which is positive-going and the other of whichis negative-going. Either output can be grounded and the other used asthe signal output, depending on the output polarity desired. Fornegative output, the positive out is grounded. The negative-going outputthen appears at the cathode of diode 74. When the magnitude of thepotential at point A drops, that is, it becomes less negative, diode 74conducts, permitting amplifier 72 to restore the proper bias. Thishappens when, for example, the trailing edge of the element 16 passesover the brush so that part of the area sensed carries no charge, andthe average potential sensed is lower than required bias. The amplifierholds the bias at the correct level until the trailing edge 34 of member20 rides past switch 30, releasing it and with it relay 46. By thistime, element 16 is well past the surface of the developing brush 10, sothere is no further need to hold the bias automatically.

In the event a positive initial polarity is to be sensed on the surfaceof insulating layer 20, switch 52 is thrown to the other position, thatis, the position indicated in dashed lines. Amplifier 64 is thus removedfrom the circuit, and a different gain-adjusting resistor 56 foramplifier 50 switched on. This is done because a different effectivegain may be needed for this operation. The negative output of amplifier62 is grounded, and the positive output fed to the anode of diode 76. Adrop in positive potential sensed by the surface of the brush reducesthe potential of the cathode of diode 76, causing it to conduct.Amplifier 72 then restores the correct bias by the same mechanismdescribed in connection with the use of a negative surface potential.

Each of diodes 74 and 76 must be capable of withstanding a high peakinverse voltage (p.i.v.) that is, with the cathode positive with respectto the anode. The p.i.v. must be at least equal to the maximum surfacepotential expected to be encountered, since, before relay 46 isactuated, the brush is at a high potential whereas the amplifier outputis essentially at ground potential. It must also have a high backresistance preferably in excess of ohms. Silicon diodes generally meetboth requirements adequately.

The operation of the invention will now be described by reference tocertain preferred embodiments thereof.

EXAMPLE 1 Negative surface potential Tabulated below are the valuesselected for proper operation of the circuit with the particularamplifiers chosen.

Resistors:

40l0 ohms 41-10 ohms 4210 ohms 54-50,000 ohm potentiometer 56-50,000 ohmpotentiometer 5847,000 ohms 6010,000 ohm potentiometer 6247,000 ohms66-l0,000 ohms 68l0,000 ohms 7050,000 ohm variable Capacitor: 48-0.0068microfarad Diodes:

742-1N629 diodes in series 76Same as 74 Amplifiers:

50--Operational amplifier Model 141A, made by Analog Devices, Inc.,Cambridge, Mass. 64-Model 111, same manufacturer 72High voltageoperational amplifier (Kepco Model OPS-2000, made by Kepco, Inc.,Flushing, NY.)

Power supplies:

The values selected for +LV and LV are respectively and volts Otheroperating potentials as specified by manufacturers Gain controlresistance 70 is initially set so that the output potential of amplifier72 is 500 volts with a 2-volt input. Gain control resistance 54 is thenadjusted to give a potential of 500 volts at the output of amplifier 72when the input potential of amplifier 50 is 5 volts, that is, the totalgain of the amplifier chain exactly compensates for the attenuation ofthe sensed potential produced by the voltage divider comprisingresistors 40 and 42. When an insulating surface bearing a pattern ofnegative charge corresponding to that produced by exposure to aphotographic positive is developed with the brush, the circuit is foundto hold the bias potential constant over the entire interval ofdevelopment. This is true for a surface potential of any value in theinterval between 150 volts and 600 volts.

EXAMPLE 2 Positive surface potential The components used are identicalto those used in sensing a negative surface potential, except, aspreviously noted, amplifier 64 and its associated resistances areswitched out of the circuit. The apparatus is used to develop a patternof positive electrostatic charge corresponding to that produced byexposure to a photographic negative, that is, one in which thebackground areas are charged instead of discharged. It is found that,using the same gain as is used in connection with Example 1, the biaspotential generated is only about one-half of that required for optimumdevelopment. To compensate for this, gain control resistance 56 isadjusted to give an overall gain of 200 or double that required inExample 1. That means that the output of amplifier 72 is 500 volts whenthe input to amplifier 50 is 2.5 volts instead of 5 volts. The circuitis found to hold the bias potential constant for sensed potentials in arange of from about volts to about 500 volts or higher.

From the foregoing, it will be apparent that the method of bias of thisinvention is not limited to use with magnetic brush development. It isequally Well suited for use with any method in which there is providedan electrode spaced in close proximity to a charge-bearing surface.Equally contemplated, therefore are such methods of development asliquid development, aerosol development, powder cloud development,cascade development, development with fur brushes including metallizedfur brushes, and the like. For example, a support having attachedthereto a plurality of non-metallic filaments such as natural orsynthetic fur materials which in turn may optionally bear a thin,adherent layer of metal may be used to form a development brush. Themethod of manufacture of such materials is disclosed in copending Millerapplication U.S. No. 9,457, filed Feb. 6', 1970, entitled Metallized FurMaterials. Their use in making development brushes is disclosed incopending Ville application U.S. Ser. No. 9,224, filed Feb. 6, 1970,entitled Development Process and Apparatus. A preferred brusharrangement utilizes two such brushes in tandem, one brush having lowelectrical conductivity and the other high conductivity. Thisarrangement is more fully disclosed in copending Miller application U.S.Ser. No. 9,225, filed Feb. 6, 1970, entitled Development Apparatus andProcess. A particularly preferred arrangement utilizes a double magneticbrush, such as for example, one of the type disclosed in copending VilleU.S. application Ser. No. 653,- 934, filed July 17, 1967, entitledElectrophotographic Developing Method and Apparatus.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

I claim:

1. In an electrographic reproduction process wherein an electrostaticcharge pattern on a surface of a photoconductive insulating layercarried by an electrically grounded conductive support is developed bymoving such surface relative to and in contact with a toner-carrying,electrically conductive, development brush which is grounded through aresistive path so that the charge produced on the brush duringdevelopment of the charge pattern acts as the instantaneous biaspotential for the brush, the improvement comprising a method ofstabilizing the bias potential on the brush during development of thecharge pattern, said method comprising the steps of:

storing an electrical signal which is proportional to the instantaneousbias potential after a portion of the charge pattern has been developed;

applying a bias potential on the brush which is proportional to thestored electrical signal to maintain the bias potential on the brushabove a minimum level during development of the remaining portion of thecharge pattern; and

reducing the resistance of said resistive path to such a level as toprevent any substantial increase in the brush potential above thatpotential applied to the brush by said applying means during thedevelopment of the remaining portion of the charge pattern.

2. In an electrographic apparatus comprising means for developing anelectrostatic charge pattern formed on a dielectric surface, suchdeveloping means including a development electrode and means forapplying toner to such surface while the development electrode is inclose proximity to such surface, the development electrode beingconnected to ground potential through a resistive path so that thecharge produced on the electrode during the development of the chargepattern acts as the instantaneous bias potential for the developmentelectrode, the improvement comprising means for stabilizing the biaspotential on the development electrode after a predetermined portion ofthe charge pattern has been developed, said stabilizing meanscomprising:

means operatively connected to the resistive path for storing a chargeproportional to the instantaneous charge produced on the developmentelectrode during development of the charge pattern;

means for disconnecting said storing means from said path after apredetermined portion of the charge pattern has been developed;

means for applying a bias potential to the development electrode whichis proportional to the level of charge stored by said storing meansimmediately prior to being disconnected from said resistive path,whereby the bias potential on the electrode is maintained above aminimum level during development of the remaining portion of the chargepattern; and

means for reducing the resistance of said resistive path upondisconnecting said storing means from said path to a level such as toprevent any substantial increase in the electrode potential above thatpotential applied to the development electrode by said biaspotential-applying means during the development of the remaining portionof the charge pattern.

3. The invention according to claim 2 wherein said storing meanscomprises a capacitor.

4. The invention according to claim 2 wherein said development electrodecomprises an electrically conductive electrographic development brush.

5. The invention according to claim 4 wherein said brush comprises amagnet.

6. The invention according to claim 4 wherein said brush comprises ametallized fur.

References Cited UNITED STATES PATENTS 2,956,487 10/1960 Giaimo 1l717.5X 3,554,161 1/1971 Blanchette ll88 3,452,185 6/1969 Hanson 32319 X3,399,338 8/1968 Burgert et a1 32322 X 3,509,448 4/1970 Bland 32322 X3,037,478 6/1962 Lace l18-637 3,599,605 8/1971 RolSton et a1. ll717.53,611,982 10/1971 COriale 1184 WILLIAM D. MA-RTIN, Primary Examiner M.SOFOCLEOUS, Assistant Examiner US. Cl. X.R.

